1. Field of the Invention
The present invention relates to an electroluminescent device (EL device) using an organic film containing an organic dye and, more particularly, to an organic EL device which realizes high-luminance electroluminescence by a combination of a plurality of organic films.
2. Description of the Related Art
In recent years, research and development of an organic EL device to be used as a display device or an illumination device have become a big concern. For example, Shogo Saito at Kyushu University has reported an EL device having an organic film two-layered structure of metal electrode/aromatic dye/polythiophene/transparent electrode (J.J. Appl. Phys. 25, L773, 1986). In this device, the thickness of an organic film is 1 .mu.m or more, and an application voltage is as high as 100 V. C. W. Tang et al. of Eastman Kodak Co. has reported an EL device having an organic film two-layered structure of Mg.Ag/Alq3/diamine/ITO (Appl. Phys. Lett., 51, p. 913, 1987). According to this report, when the thickness of organic film is 100 nm or less, an EL device which can be driven with an application voltage of 10 V or less to exhibit a practically satisfactory luminance is obtained. These EL devices are based on an organic two-layered structure obtained by combining a dye having an electron injecting property and a dye having a hole injecting property and have characteristic features in that the thickness of an organic film is minimized, a material having a small work function is selected as a metal electrode at an electron injection side, and an organic material is selected so as not to produce an electrical defect when an organic film is formed by a vacuum deposition method or a sublimation method. Shogo Saito at Kyushu University has further proposed an organic three-layered structure of electron injection layer/electroluminescent layer/hole injection layer. This report reveals that high-luminance electroluminescence can be obtained by this structure when a dye having high photoluminescence is selected as the electroluminescent layer (J.J. Appl. Phys., 27, L269, 1988).
In addition, many researches such as a research of an electroluminescent device structure obtained by combining various types of organic films, a research indicating that a certain degree of electroluminescence can be obtained even in a single-layered organic film by mixing an electroluminescent agent and a hole injecting agent, and a research concerning characteristic degradation in Alq3 as an electroluminophor have been reported, and a large number of associated patent applications have been filed.
The organic EL device has almost reached the stage of practical applications in terms of a luminance of electroluminescence but still has many technically unsolved problems in, e.g., a luminous efficacy, a device life, and a device formation process. For example, a luminous efficacy of the device is currently at most 1% and is normally about 0.1%. The low luminous efficacy indicates that a current not contributing to electroluminescent flows between electrodes. This current is a serious cause of a short device life since it generates Joule heat. In order to put the organic EL device into practical use, therefore, the luminous efficacy is desirably increased to at least several to ten percents.
Each organic film of the organic EL device normally consists of a single dye. Since such a dye has an absorption edge from visible to ultraviolet ranges, its band gap is 1.5 eV or more, and almost no carriers are present in each organic film at room temperature. Carriers, therefore, are supplied from metal electrodes to the organic film. In addition, the mobility of carrier moving in the organic dye is generally small. For these reasons, the electrical resistance of the organic dye film is very high. For example, in a device of Tang et al., a current of 10 to 100 mA/cm.sup.2 flows with an application voltage of 10 V. In this case, the resistance of the organic film is 5.times.10.sup.6 to 10.sup.7 .OMEGA./cm.sup.2. Since the thickness of the organic film is 0.2 mm, its resistivity is as very high as 5.times.10.sup.6 to 10.sup.7 .OMEGA..multidot.cm. This high resistance characteristic of the organic film causes a voltage drop of the device and generation of Joule heat and therefore reduces a luminous efficacy or shortens a device life.
For the above reasons, in order to increase the luminous efficacy of the organic EL device, a device structure and electrical properties of a material to be used must be optimized. Since, however, only qualitative properties such as an electron (hole) transport property, an electron (hole) injection property, an electron donor property, an electron acceptor property, and an electroluminescent property have been defined as the properties of an organic material, device conditions are not satisfactorily defined.